In the laying up of laminates of composite material, it is usually necessary to stop after each several layers, or sometimes after each layer, and compact the material. This is required in order to remove air pockets and to force the layers to conform to the mold surface on which they have been laid. When the lay-up has been completed, it is normally cured at an elevated pressure and temperature in an autoclave. The current practice is to employ a reusable silicone rubber blanket in both the compacting and the curing of the laminate. The blanket is stretched over a seal that surrounds the area of the lay-up structure on which the laminate is being laid up, and a vacuum is applied inside the seal between the blanket and the lay-up surface. During compaction, the vacuum causes the blanket to exert pressure on the layers of the laminate to compact the layers. During the curing cycle, the autoclave is pressurized to increase the force on the layers of the laminate.
The need for carrying out repeated compacting steps adds to the time and labor necessary to form a laminate component. The increased time and effort required tends to make composite material components relatively expensive to manufacture. Therefore, it is highly desirable to carry out the compacting steps as quickly and easily as possible in order to minimize the cost of the operation. One step of the compacting procedure that has been cumbersome and time consuming using known techniques is the attaching of the blanket to the lay-up structure to ensure that it is and remains positioned properly with respect to the seal and the laminate during the rest of the compacting procedure.
In a possible scheme for removably attaching the blanket, a number of metal grommets are bonded to the rubber blanket at spaced intervals around the edge of the blanket. An anchor screw bracket attached to the lay-up structure is provided for each of the grommets. Experiments have shown this scheme to have a number of disadvantages. The attaching of the blanket is time consuming because of the need to align the anchor screws with the grommets when the blanket is laid over the work surface. There is a concentration of stress on the blanket around each of the grommets, and therefore, the useful life of the blanket is likely to be shortened. The need to replace the blanket more often would further add to the cost of the manufacture of the composite material components. In addition, there is no provision for adjusting the tension on the blanket to compensate for shrinkage or stretching of the blanket and avoid premature replacement of the blanket.
Another scheme that has been tried is to attach a material such as the material sold under the trademark Velcro to the edges of the blanket and to the structure outside the periphery of the seal. This method has proved inadequate because the Velcro material cannot tolerate high autoclave temperatures and fails after a few cycles and because the rigidity of Velcro material tape is incompatible with flexible rubber blankets. Zippers have also been tried, but proved inadequate when there was shrinkage of the rubber blanket due to high temperatures. Still another method that has been tried is to use a "picture frame" with the rubber blanket stretched over or under it. This technique works on small, relatively flat parts, but becomes very unwieldly for large parts and parts with extreme contours.
Each of the following United States patents discloses a vacuum bagging or other process in which a flexible sheet is urged against a workpiece by applying a vacuum: U.S. Pat. No. 2,978,806, granted Apr. 11, 1961, to T. A. Herbert, Jr.; U.S. Pat. No. 3,666,600, granted May 30, 1972, to S. Y. Yoshino; U.S. Pat. No. 3,912,542, granted Oct. 14, 1975, to J. Hirano et al; and U.S. Pat. No. 4,287,015, granted Sept. 1, 1981, to H. J. Danner, Jr. Herbert discloses a flexible sheet that is looped around its edges. Cylindrical weights are received into the loops formed by the flexible sheet and hang down over the edges of the work surface to apply tension to the flexible sheet. Yoshino does not discuss the attaching of the flexible sheet to the mold structure. However, it appears that the sheet simply adheres to the zinc chromate putty seal. In the Hirano et al method, the edges of the sheet are held down by a rectangular frame which is clamped to the work table. Danner discloses attaching the flexible cover sheet with reusable fasteners, such as a zipper, clamps, or Velcro.
U.S. Pat. No. 3,632,730, granted Jan. 4, 1972, to J. E. Cotton discloses a process in which separate sections of a semi-tubular flume are formed together in a nested configuration over a domed mold. A number of laterally spaced straps are placed over the layers of reinforced cement to be formed. A lever arrangement tightens the straps by winding the ends of the straps about a shaft positioned under the mold.
The patents described above, together with the prior art cited in the patents, should be carefully considered for the purpose of putting the present invention into proper perspective relative to the prior art.